1. Field of the Invention
The invention relates to a charging device for charging of rechargeable NiCd, Ni-metal hydride or lithium batteries of implants (with one or more cells or with one or more secondary elements, as required) by transcutaneous transmission of electric power from an external power transmission part to a power receiving part which forms a part of the implant, the charging device being provided with a charging current detector which in a first charging phase allows a relatively high charging current to flow and which, after the cell voltage of the battery has reached a predetermined limiting charging voltage, in a second charging phase reduces the charging current compared to the charging current which flows at the end of the first charging phase. The invention furthermore relates to a process for charging of implanted batteries of implants by transcutaneous transmission of electric power in which, in the first charging phase, a relatively high charging current flows, and in which, after the cell voltage of the battery has reached a predetermined limiting charging voltage, in a second charging phase, the charging current is reduced compared to the charging current flowing at the end of the first charging phase.
2. Description of Related Art
When a battery is charged, only one part of the supplied electric power is converted into charge. Another part of this power is converted into heat on the internal resistance of the battery and is lost for charging. The power loss can lead to an impermissible temperature rise of the implant housing, and thus, to damage of the surrounding tissue.
Another part of the supplied energy drives secondary electrochemical reactions which, for example, lead to gas evolution within the battery. This applies mainly when the battery has reached a higher charging level, for example, is charged to more than 80% of its nominal capacity. Especially over years of operation of an implanted battery does the capacity ratio of the positive and negative electrodes of the battery cell or cells shift due to electrolyte loss and passivation of the electrode surfaces and/or by corrosion with time, such that, during charging (i.e., re-charging), a greater and greater preponderance of the gas-forming over the gas-consuming reactions occurs, and thus, the internal pressure of the cell rises quickly during charging. As the gas pressure rises the cell housing swells, which under certain circumstances can lead to destruction of the cell or the implant. The increasing corrosion and/or passivation of the electrodes and the decrease of the electrolyte-wetted electrode surface, at the same time, cause an increase of the internal resistance of the battery.
In one such charging device and a charging process of the indicated type, use of a charging protocol is known (U.S. Pat. Nos. 5,690,693; 5,702,431 and 5,411,537) which provides for either the instantaneous charging current or the duty factor of a fixed charging current to be reduced as the charging level of the battery increases. Thus, in U.S. Pat. No. 5,411,537, a multistage charging process for lithium batteries is proposed in which, during the charging process, the no-load voltage of the battery is periodically interrogated to estimate the charging state of the battery, and in which, depending on the no-load voltage determined at the time, one of several predetermined charging current values is selected, the charging current values becoming smaller as the no-load voltage increases. The charging process is ended when the measured no-load voltage exceeds a stipulated boundary value.
U.S. Pat. No. 5,702,431 discloses an alternative of the above described multistage charging process in which the amplitude of the charging current remains constant, but the charging current is pulse width modulated and the duty factor is changed depending on the periodically determined no-load voltage values. Similarly, in U.S. Pat. No. 5,690,693, a pulse width modulation device with a variable duty cycle is used as a controller for the current level applied during the charging process.